Zn-Alloyed All-Inorganic Halide Perovskite-Based White Light-Emitting Diodes with Superior Color Quality

Highly Photostable Zn-Treated Halide Perovskite Nanocrystals for Efficient Single Photon Generation

Achieving pure single-photon emission is essential for a range of quantum technologies, from quantum computing to quantum key distribution to quantum metrology. Among solid-state quantum emitters, colloidal lead halide perovskite (LHP) nanocrystals (NCs) have attracted considerable interest due to their structural and optical properties, which make them attractive candidates for single-photon sources (SPSs). However, their practical utilization has been hampered by environment-induced instabilities. In this study, we fabricate and characterize in a systematic manner Zn-treated CsPbBr3 colloidal NCs obtained through Zn2+ ion doping at the Pb-site, demonstrating improved stability under dilution and illumination. The doped NCs exhibit high single-photon purity, reduced blinking on a submillisecond time scale, and stability of the bright state even at excitation powers well above saturation. Our findings highlight the potential of this synthesis approach to optimize the performance of LHP-based SPSs, opening up interesting prospects for their integration into nanophotonic systems for quantum technology applications.

Bluish-white Light-emitting 2D Sheets of Lead-free Perovskite Cesium Titanium Bromide (CsTiBr3) by a Two-stage Deposition Technique

Bluish-white light-emitting materials are commonly used in LED lighting because they produce natural-looking light. Here we report the photoluminescent emission (PL) of novel, two-dimensional lead-free CsTiBr3 perovskite prepared via a two-stage deposition process. The formation of two-dimensional nanosheets of CsTiBr3 perovskite is confirmed by XRD, EDAX, and FESEM analysis. The height of the cesium bromide thin film substrate from the titanium bromide vapor source plays an important role in the formation of two-dimensional CsTiBr3. The CsTiBr3 perovskite nanosheets exhibit unique exciton- luminescence at 440 nm and self-trapped exciton emission at 595 nm which are the characteristics of two-dimensional halide structure, along with the band-to-band emission at 400 nm at an excitation wavelength of 340 nm. The resulting bluish-white light PL emission makes two-dimensional CsTiBr3 perovskite an alternative material to the traditional lead-based perovskite in LEDs, display technology, solid-state lighting, and various optoelectronic devices, addressing environmental concerns.

Advances in All-Inorganic Perovskite Nanocrystal-Based White Light Emitting Devices

Metal halide perovskites (MHPs) are exceptional semiconductors best known for their intriguing properties, such as high absorption coefficients, tunable bandgaps, excellent charge transport, and high luminescence yields. Among various MHPs, all-inorganic perovskites exhibit benefits over hybrid compositions. Notably, critical properties, including chemical and structural stability, could be improved by employing organic-cation-free MHPs in optoelectronic devices such as solar cells and light-emitting devices (LEDs). Due to their enticing features, including spectral tunability over the entire visible spectrum with high color purity, all-inorganic perovskites have become a focus of intense research for LEDs. This Review explores and discusses the application of all-inorganic CsPbX₃ nanocrystals (NCs) in developing blue and white LEDs. We discuss the challenges perovskite-based LEDs (PLEDs) face and the potential strategies adopted to establish state-of-the-art synthetic routes to obtain rational control over dimensions and shape symmetry without compromising the optoelectronic properties. Finally, we emphasize the significance of matching the driving currents of different LED chips and balancing the aging and temperature of individual chips to realize efficient, uniform, and stable white electroluminescence.

Transforming exciton dynamics in perovskite nanocrystal through Mn doping

Zn-alloyed CsPb(Cl/Br)₃ perovskite nanocrystals (PNCs) have been synthesized and used as a model system for Mn doping in order to understand the effect of Mn doping on exciton dynamics. While keeping the PL emission maximum and PLQY of both PNC samples nearly the same, the radiative decay rate of the host band decreases ∼6.5 times and the non-radiative decay rate increases ∼2.5 times upon Mn doping. Unlike reports in the literature in which the dopant emission decreases to near-zero, in the present case we observe ∼5.5-fold enhancement of the integrated PL intensity of the dopant emission when the temperature decreases from 290 K to 190 K. Interestingly, the FWHM of the host PL emission band increases with a decrease in temperature from 290 K to 190 K. A higher value of phonon energy in PNC2 (58 ± 2 meV) in comparison to CsPbBr₃ has been noted. The low magnitude of the Huang-Rhys factor indicates less electron phonon coupling for the Mn-doped PNC system. Temperature-dependent dopant PL decay exhibits biexponential decay behaviour with time constants τ₁ = 450-540 μs and τ₂ = 1.1-1.2 ms. With a decrease in temperature from 290 K to 190 K, the amplitude of the faster component decreases from 80% to 60%; concomitantly, the amplitude of the slower component increases from 20% to 40%. Ultrasensitive single-particle spectroscopic analyses reveal that, although the probability density distributions (PDDs) of the durations of both ON and OFF events of PNC1 could be fitted with a truncated inverse power law (TIPL), however, for PNC2, both PDDs could be fitted with an inverse power law (IPL). A comparatively lower value of the power law exponent m_ON indicates a higher probability of longer ON events for PNC1 than for PNC2. Truncation in the PDDs of both ON and OFF events has been observed for PNC1, but not in the PDDs of either ON or OFF events for PNC2. The presence of shallow trap states is responsible for the truncation for PNC1, whereas the presence of deep dopant states does not allow truncation in the host PL emission of PNC2. All these observations clearly demonstrate that Mn doping transforms the host PL exciton dynamics for Zn-alloyed Mn-doped CsPb(Cl/Br)₃ PNCs very significantly.

Controlled Assembly and Anomalous Thermal Expansion of Ultrathin Cesium Lead Bromide Nanoplatelets

Quantum confined lead halide perovskite nanoplatelets are anisotropic materials displaying strongly bound excitons with spectrally pure photoluminescence. We report the controlled assembly of CsPbBr₃ nanoplatelets through varying the evaporation rate of the dispersion solvent. We confirm the assembly of superlattices in the face-down and edge-up configurations by electron microscopy, as well as X-ray scattering and diffraction. Polarization-resolved spectroscopy shows that superlattices in the edge-up configuration display significantly polarized emission compared to face-down counterparts. Variable-temperature X-ray diffraction of both face-down and edge-up superlattices uncovers a uniaxial negative thermal expansion in ultrathin nanoplatelets, which reconciles the anomalous temperature dependence of the emission energy. Additional structural aspects are investigated by multilayer diffraction fitting, revealing a significant decrease in superlattice order with decreasing temperature, with a concomitant expansion of the organic sublattice and increase of lead halide octahedral tilt.

Sr-Doping All-Inorganic CsPbBr3 Perovskite Thick Film for Self-Powered X-ray Detectors

The all-inorganic perovskite cesium lead bromine (CsPbBr₃) has attracted much attention in the field of X-ray detectors because of its high X-ray absorption coefficient, high carrier collection efficiency, and easy solution preparation. The low-cost anti-solvent method is the main method to prepare CsPbBr₃; during this process, solvent volatilization will bring a large number of holes to the film, leading to the increase of defects. Based on the heteroatomic doping strategy, we propose that Pb²⁺ should be partially replaced by Sr²⁺ to prepare leadless all-inorganic perovskite. The introduction of Sr²⁺ promoted the ordered growth of CsPbBr₃ in the vertical direction, increased the density and uniformity of the thick film, and achieved the goal of CsPbBr₃ thick film repairing. In addition, the prepared CsPbBr₃ and CsPbBr₃:Sr X-ray detectors were self-powered without external bias, maintaining a stable response during on and off states at different X-ray dose rates. Furthermore, the detector base on 160 µm CsPbBr₃:Sr had a sensitivity of 517.02 µC Gy_air^-1 cm^-3 at zero bias under the dose rate of 0.955 µGy ms^-1 and it obtained a fast response speed of 0.053-0.148 s. Our work provides a new opportunity to produce cost-effective and highly efficient self-powered perovskite X-ray detectors in a sustainable way.

Synthesis and Applications of Halide Perovskite Nanocrystals in Optoelectronics

The perovskites used for optoelectronic devices have been more attractive during recent years due to their wide variety of advantages, such as their low cost, high photoluminescence quantum yield (PLQY), high carrier mobility, flexible bandgap tunability, and high light absorption ability. However, optoelectronic applications for traditional inorganic and organic materials present dilemmas due to their hardly tunable bandgap and instability. On the other hand, there are some more important benefits for perovskite nanocrystals, such as a size-dependent bandgap and the availability of anion exchange at room temperature. Therefore, perovskite NC-based applications are currently favored, offering a research direction beyond perovskite, and much research has focused on the stability issue and device performance. Thus, the synthesis and applications of perovskite NCs need to be thoroughly discussed for the future development of solar cells, light-emitting diodes, photodetectors, and laser research.

A Low-Cost Synthetic Route of FAPbI3 Quantum Dots in Air at Atmospheric Pressure: The Role of Zinc Iodide Additives

Perovskite quantum dots (PQDs) have shown great promise in optoelectronic device applications. Typically, a traditional hot-injection method with heating and high vacuum pressure is used to synthesize these colloidal nanoparticles. In this article, we report a low-cost synthetic method for FAPbI₃ PQDs in air at atmospheric pressure with the assistance of ZnI₂. Compared with the FAPbI₃ PQDs synthesized under vacuum/N₂ condition, the air-synthesized Zn:FAPbI₃ PQDs exhibit the same crystalline structure with a similar preferential crystallographic orientation but demonstrate higher colloidal stability and higher production yield. Furthermore, we examine the influence of ZnI₂ during the synthesis process on morphologies and optoelectronic properties. The results show that the mean size of the obtained FAPbI₃ PQDs is decreased by increasing the amount of added ZnI₂. More importantly, introducing an optimal amount of ZnI₂ into the Pb source precursor enables increasing the carrier lifetime of FAPbI₃ PQDs, showing the potential beneficial effect on device performance.

Self-Assembly of Solvent-Stabilized Au Nanocluster as Efficient Förster Resonance Energy-Transfer Initiator for White Light Generation

Aggregation-induced enhancement (AIE) in the photoluminescence quantum yield (PLQY) from 12.5 to 51% in the N,N-dimethylformamide (DMF)-stabilized Au nanocluster (AuNC) system is reported here. The self-assembling of AuNC has been achieved via hydrogen bonding interaction, which is further utilized in designing the AuNC_DCM system for realizing a Förster resonance energy transfer (FRET)-based white LED (WLED), having CIE coordinates of (0.35, 0.29). The solution-processed fabrication strategy used, has given us the liberty to optimize its components for optimal full-spectrum light output. The CIE coordinates of the designed WLED have been improved further to (0.33, 0.32), with a high color rendering index of 93 and correlated color temperature of 5620 K by incorporating a green emitter, namely nitrogen-doped graphene quantum dots (NGQD), in the AuNC_DCM system. The excellent spectral quality of the as-designed WLED and the repeatability of the proposed fabrication method will make the developed AuNCs_DCM FRET conjugate useful in practical photonic applications.

Wide-Bandgap Perovskite Quantum Dots in Perovskite Matrix for Sky-Blue Light-Emitting Diodes

The epitaxial growth of a perovskite matrix on quantum dots (QDs) has enabled the emergence of efficient red light-emitting diodes (LEDs) because it unites efficient charge transport with strong surface passivation. However, the synthesis of wide-band gap (E_g) QD-in-matrix heterostructures has so far remained elusive in the case of sky-blue LEDs. Here, we developed CsPbBr₃ QD-in-perovskite matrix solids that enable high luminescent efficiency and spectral stability with an optical E_g of over 2.6 eV. We screened alloy candidates that modulate the perovskite E_g and allow heteroepitaxy, seeking to implement lattice-matched type-I band alignment. Specifically, we introduced a CsPb_1-xSr_xBr₃ matrix, in which alloying with Sr²⁺ increased the E_g of the perovskite and minimized lattice mismatch. We then developed an approach to passivation that would overcome the hygroscopic nature of Sr²⁺. We found that bis(4-fluorophenyl)phenylphosphine oxide strongly coordinates with Sr²⁺ and provides steric hindrance to block H₂O, a finding obtained by combining molecular dynamics simulations with experimental results. The resulting QD-in-matrix solids exhibit enhanced air- and photo-stability with efficient charge transport from the matrix to the QDs. LEDs made from this material exhibit an external quantum efficiency of 13.8% and a brightness exceeding 6000 cd m^-2.

Yb2+-Alloyed Cs4PbI6-CsPbI3 Perovskite Nanocomposites for Efficient and Stable Pure-Red Emission

A series of Yb²⁺-alloyed CsPb_1-xYb_xI₃ (x = 0, 0.2, 0.4, 0.6) perovskite nanocrystals (NCs): are synthesized by a modified hot-injection method. Yb²⁺ alloying induced a blue shift of photoluminescence (PL) spectra. In particular, when x = 0.6, the perovskite NCs exhibit pure-red emission with PL centered at 638 nm. Furthermore, the perovskite NCs with pure-red emission exhibit enhanced air and thermal stability, compared to pure CsPbI₃ NCs. The enhanced stability can be assigned to the formation Cs₄PbI₆-CsPbI₃:Yb composites. Charge-carrier dynamics study indicates that the Cs₄PbI₆-CsPbI₃:Yb composites exhibit ultrafast hot-carrier cooling processes, which could break the Auger reheating effect. These properties suggest the Yb²⁺ alloyed CsPbI₃ perovskite NCs have great potential for high-performance pure-red light-emitting diodes.

Tunable White Light-Emitting Devices Based on Unilaminar High-Efficiency Zn2+-Doped Blue CsPbBr3 Quantum Dots

Perovskite-based white-light-emitting devices (WLEDs) are expected to be the potential candidate for the next-generation lighting field due to their scalability and low-cost process. However, simple and adjustable WLED fabrication technology is in urgent need. Here, WLEDs with a single layer of perovskite quantum dots (PQDs) were constructed by combining Zn²⁺-doped CsPbBr₃ PQDs with exciplex emission between poly(9-vinylcarbazole) (PVK) and ((1-phenyl-1H-benzimidazol-2-yl)benzene)) (TPBi). Zn²⁺-doped CsPbBr₃ PQDs with polar ion shells were prepared by means of low temperature and post-treatment. The photoluminescence quantum yield (PLQY) can reach as high as 95.9% at the emission wavelength of 456 nm. The blue shift of its PL (∼60 nm) is much greater than that of other reported Zn²⁺-doped CsPbBr₃ PQDs (5-10 nm), thus realizing the true blue-emission Zn²⁺-doped CsPbBr₃ PQDs. As a result, just by controlling the thickness of TPBi, the adjustment of cold (CIE (0.2531, 0.2502)) and warm WLEDs (CIE (0.3561, 0.3562)) is realized for the first time.

Near Unity PLQY and High Stability of Barium Thiocyanate Based All-Inorganic Perovskites and Their Applications in White Light-Emitting Diodes

All-inorganic lead halide perovskite (CsPbX3) nanocrystals (NCs) have emerged as a highly promising new generation of light emitters due to their extraordinary photophysical properties. However, the performance of these semiconducting NCs is undermined due to the inherent toxicity of lead and long-term environmental stability. Here, we report the addition of B-site cation and X-site anion (pseudo-halide) concurrently using Ba(SCN)2 (≤50%) in CsPbX3 NCs to reduce the lead and improve the photophysical properties and stability. The as-grown particles demonstrated an analogous structure with an almost identical lattice constant and a fluctuation of particle size without altering the morphology of particles. Photoluminescence quantum yield is enhanced up to near unity (~98%) by taking advantage of concomitant doping at the B- and X-site of the structure. Benefitted from the defect reductions and stronger bonding interaction between Pb2+ and SCN− ions, Ba(SCN)2-based NCs exhibit improved stability towards air and moisture compared to the host NCs. The doped NCs retain higher PLQY (as high as seven times) compared to the host NCs) when stored in an ambient atmosphere for more than 176 days. A novel 3D-printed multiplex color conversion layer was used to fabricate a white light-emitting diode (LED). The obtained white light shows a correlated color temperature of 6764 K, a color rendering index of 87, and luminous efficacy of radiation of 333 lm/W. In summary, this work proposes a facile route to treat sensitive lead halide perovskite NCs and to fabricate LEDs by using a low-cost large-scale 3-D printing method, which would serve as a foundation for fabricating high-quality optoelectronic devices for near future lighting technologies.

Lead-Free Halide Double Perovskite Nanocrystals for Light-Emitting Applications: Strategies for Boosting Efficiency and Stability

Lead-free halide double perovskite (HDP) nanocrystals are considered as one of the most promising alternatives to the lead halide perovskite nanocrystals due to their unique characteristics of nontoxicity, robust intrinsic thermodynamic stability, rich and tunable optoelectronic properties. Although lead-free HDP variants with highly efficient emission are synthesized and characterized, the photoluminescent (PL) properties of colloidal HDP nanocrystals still have enormous challenges for application in light-emitting diode (LED) devices due to their intrinsic and surface defects, indirect band, and disallowable optical transitions. Herein, recent progress on the synthetic strategies, ligands passivation, and metal doping/alloying for boosting efficiency and stability of HDP nanocrystals is comprehensive summarized. It begins by introducing the crystalline structure, electronic structure, and PL mechanism of lead-free HDPs. Next, the limiting factors on PL properties and origins of instability are analyzed, followed by highlighting the effects of synthesis strategies, ligands passivation, and metal doping/alloying on the PL properties and stability of the HDPs. Then, their preliminary applications for LED devices are emphasized. Finally, the challenges and prospects concerning the development of highly efficient and stable HDP nanocrystals-based LED devices in the future are proposed.

Impact of Hydroiodic Acid on Resistive Switching Performance of Lead-Free Cs3Cu2I5 Perovskite Memory

Herein, we employed lead-free Cs₃Cu₂I₅ perovskite films as the functional layers to construct Al/Cs₃Cu₂I₅/ITO memory devices and systematically investigated the impact on the corresponding resistive switching (RS) performance via adding different amounts of hydroiodic acid (HI) in Cs₃Cu₂I₅ precursor solution. The results demonstrated that the crystallinity and morphology of the Cs₃Cu₂I₅ films can be improved and the resistive switching performance can be modulated by adding an appropriate amount of HI. The obtained Cs₃Cu₂I₅ films by adding 5 μL HI exhibit the fewest lattice defects and flattest surface (RMS = 13.3 nm). Besides, the memory device, utilizing the optimized films, has a low electroforming voltage (1.44 V), a large on/off ratio (∼65), and a long retention time (10⁴ s). The RS performance impacted by adding HI, providing a scientific strategy for improving the RS performance of iodine halide perovskite-based memories.

Enhanced luminescence through interface energy transfer in hierarchical heterogeneous nanocomposites and application in white LEDs

Highly efficient light-emitting materials are essential for achieving high-performance devices. Here, a novel composite system, as well as enhanced luminescence processes, was designed, where NaLn(MoO₄)₂ ultra-small nucleus can be effectively isolated by In(OH)₃ to form NaLn(MoO₄)₂@In(OH)₃ composite nanoclusters due to the different nucleation rate between NaLn(MoO₄)₂ and In(OH)₃, and then these small composite clusters gradually self-assemble into hierarchical structures. As we expected, the enhanced luminescence was achieved from hierarchical NaLn(MoO₄)₂ nanostructures with adjusting the distance among NaLn(MoO₄)₂ ultra-small nucleus by inserting In(OH)₃. A series of spectroscopy results show that the In(OH)₃ not only acts as an energy transfer bridge from CTB Eu³⁺ → O^2- (or MoO₄^2- absorption) to Eu³⁺, but also can effectively alleviate the concentration quenching of Ln³⁺ and change the J-O parameters. The Raman peak at 134 cm^-1 is helpful to populate the ⁵D₀ level of Eu³⁺ or the excited states of Er³⁺, resulting in stronger up/down-conversion emissions. The use of NaLn(MoO₄)₂@In(OH)₃ in white light-emitting diodes (LEDs) has been demonstrated. The combination of red emission from NaLn(MoO₄)₂@In(OH)₃ with blue, green, and yellow emission from halide perovskites could achieve white light with excellent vision performance (an LER of 376 lm/W) and superior color quality (CRI > 92). The findings of this experiment provide a new idea for the design of composite interface materials.

CsCu2I3 Nanocrystals: Growth and Structural Evolution for Tunable Light Emission

CsCu₂I₃ mixed with Cs₃Cu₂I₅ has shown potential applications as white-light-emitting materials, while their growth, structural evolution behaviors, and their impact on photoluminescence of CsCu₂I₃ nanocrystals (NCs) are still not known. In this work, we investigated the growth and structural evolution of CsCu₂I₃ nanocrystals with increasing reaction temperature. At low temperature and in the presence of a high dosage of oleic acid and oleylamine, Cs₃Cu₂I₅ nanoparticles, rather than CsCu₂I₃ NCs, preferred to form in the hot-injection reaction system. Increasing the reaction temperature promoted the formation of CsCu₂I₃ nanorods. Phase-pure CsCu₂I₃ nanorods were steadily obtained at 180 °C. Structural evolution from less copper-containing NCs to copper-rich ones in the low-temperature reaction condition is highly related to the coordination of copper ions with OAm. More importantly, accompanying the growth of nanorods and structural evolution from Cu₃Cs₂I₅ to CsCu₂I₃, the color of photoluminescence emission of NCs changed from blue to nearly white and to yellow, but their photoluminescence quantum yield decreased from 36.00 to 9.86%. The finding in this work would give a view to the structural evolution of copper-containing perovskite-like halides, being helpful for adjusting their photoluminescence in white LEDs.

Emergence of Impurity-Doped Nanocrystal Light-Emitting Diodes

In recent years, impurity-doped nanocrystal light-emitting diodes (LEDs) have aroused both academic and industrial interest since they are highly promising to satisfy the increasing demand of display, lighting, and signaling technologies. Compared with undoped counterparts, impurity-doped nanocrystal LEDs have been demonstrated to possess many extraordinary characteristics including enhanced efficiency, increased luminance, reduced voltage, and prolonged stability. In this review, recent state-of-the-art concepts to achieve high-performance impurity-doped nanocrystal LEDs are summarized. Firstly, the fundamental concepts of impurity-doped nanocrystal LEDs are presented. Then, the strategies to enhance the performance of impurity-doped nanocrystal LEDs via both material design and device engineering are introduced. In particular, the emergence of three types of impurity-doped nanocrystal LEDs is comprehensively highlighted, namely impurity-doped colloidal quantum dot LEDs, impurity-doped perovskite LEDs, and impurity-doped colloidal quantum well LEDs. At last, the challenges and the opportunities to further improve the performance of impurity-doped nanocrystal LEDs are described.

Dual-Mode Light-Emitting Lanthanide Metal-Organic Frameworks with High Water and Thermal Stability and Their Application in White LEDs

It is well known that the upconversion luminescence from lanthanide metal-organic frameworks (Ln-MOFs) is difficult to achieve, and thus, there are few reports on dual luminescence-based MOFs. Here, dual-mode light-emitting Ln-MOFs are synthesized using a low-cost hydrothermal method. Our results show that the obtained Ln-MOFs not only have high thermal stability (up to 420°) but also are stable in deionized water. The dual-mode up- and downconversion luminescence is simultaneously observed from Er-Eu-MOFs. The temperature-dependent fluorescence decay time is calculated to be ranging from 0.46 to 0.36 ms for temperatures from 100 to 300 K. We suggested that this phenomenon was because the number of phonons participating in the MOF matrix increases with temperature during the luminescence process, and the phonons interact with the electrons in the material. The values of the J-O parameters calculated from the emission spectra indicated that the symmetry around Eu³⁺ ions in Eu-MOF is the highest, which was also higher than that of Er-Eu-MOF. To explore the potential applications of Eu-MOFs in white light-emitting diodes (LEDs), red emission from Eu-MOFs was combined with blue, green, and yellow emissions from metal halide perovskites to achieve white light emission. White light with excellent color quality and vision performance was obtained. These findings demonstrate that Ln-MOFs are potentially successful materials for applications in white LEDs.